Superconducting magnets and inductors are capable of generating high magnetic fields and thereby storing large amounts of energy. Superconducting inductors are very efficient for these purposes because no energy is lost to resistive or joule heating in the superconducting current path. Every superconducting material has a critical temperature T.sub.c for a given ambient magnetic field above which the material is no longer superconducting. If a region of a superconducting conductor loses its superconducting property (i.e., becomes normal or quenches) while current is flowing in the conductor, joule heating occurs in the normal or nonsuperconducting region. If the region is small enough, the small amount of heat will be dissipated and the region will return to its superconducting state.
If the region is large, such that sufficient joule heating occurs and overcomes the system's ability to dissipate the heat, the normal zone will propagate and grow larger, causing a catastrophic condition which can result in severe damage to the inductor or magnet, as even more energy is dissipated in portions of the inductor or magnet. This runaway condition can result in the uncontrolled dumping of the entire magnetic energy of the inductor or magnet, causing damage to itself and possibly to the load. It could also result in a service outage of the inductor or magnet, which could be intolerable if the inductor or magnet where part of a device having a critical military application. Early detection of a quench, however, permits the energy stored in the magnet or inductor to be dissipated in a controlled fashion. The energy can be dissipated in a variety of ways such as through dump resistors or by making the entire magnet or inductor go normal. Catastrophic physical damage due to overheating is therefore avoided.
Generally, a superconducting inductor is any current path composed of a superconducting material since any current path has a self-inductance. Superconducting inductors, and especially superconducting energy storage inductors, are generally configured as coils called solenoids or toroids. Superconducting magnets, which are a special case of superconducting inductors, often have more complex shapes, so as to appropriately shape the magnetic field they produce.
Several techniques exist for detecting and locating normal regions in a superconducting magnet or inductor. The principle technique has been the use of a series of voltage taps. Voltages are measured by voltmeters at various points along the coil of the superconducting material, with the objective of correlating changes in voltage caused by the change in resistivity due to the creation of a normal region. A severe drawback with using voltage taps is that in addition to the resistive voltage associated with a normal zone, a superconducting inductor produces inductive voltages resulting from the charging and discharging of the coil. These "common mode" inductive voltages are variable and change with any changes in the magnetic field. Also, since normal zones must be detected when they are small, the resistive voltage resulting from a quench is very small, typically less than a volt; whereas the common mode inductive voltage between two voltage taps is typically much larger and can be tens of kilovolts. If voltage taps are used, some technique must be utilized to eliminate the inductive voltage from the voltage measured at the taps. Typically, this involves subtracting out the inductive voltage by comparing the signal to a reference voltage. The reference voltage can be from a sense coil monitoring the magnetic field, or may be a voltage taken from a voltage tap in a different part of the inductor. In any event, the technique involves subtracting two voltage measurements, one entirely inductive and the other mostly inductive, but also having a small resistive component with the hope of recovering that small resistive component. It is understood that "inductive" as used here means that the phase of the voltage is that phase which would be produced by a pure inductor, i.e., the voltage is proportional to the time rate of change of the current. The entirely inductive voltage could be derived by other means, such as an electronic differentiating machine.
As mentioned above, the inductive voltage across the inductor's terminals may be tens of kilovolts during normal operation. This means the sensor must be floated at these high voltages. This places a severe constraint upon any electronic components attached to the voltage taps to measure and detect a quench since these components must be designed to operate at these high inductive voltages. Also, since the components must be placed close to the voltage taps, they must be capable of operating properly at cryogenic temperatures and in high magnetic fields.
It would be desirable, therefore, to develop a sensor for quench detection in superconductors under operating and quiescent conditions wherein a quench can easily be detected without encountering the difficulties mentioned above.